According to GB No. 2,131,435A, synthetic elastomers, e.g., ethylenepropylene terpolymers (EPDM), styrene-butadine rubber (SBR), and butyl and nitrile rubbers, find widespread use, for instance in sealants, gaskets, conveyor belts and electrical cable insulation. In many of these applications they are preferred to natural rubber because of their superior resistence to degradation by heat, oils, solvents, oxygen and ozone.
Most synthetic elastomers belong to the non-crystallyzing category and, unlike natural rubber, their gum strength is poor. Useful mechanical properties can be achieved, however, by the addition of a particulate filler that improves the modulus and failure properties of the cured vulcanisate. This phenomenon is known as reinforcement and is associated with an increase in modulus, tensile strength and swelling resistance. Significant reinforcement is, in general, manifested only when the filler particles have a high surface area and the elastomer wets the filler.
Hitherto the most important and commonly used reinforcing filler has been carbon black. The so-called structured or reinforcing grades of carbon black are prepared from relatively expensive hydrocarbon feedstocks such as the aromatic fraction of petroleum or natural gas by burning in an oxygen-depleted atmosphere in special furnaces. Moreover, these feedstocks are in demand for the production of monomers (e.g styrene) and other chemicals. Consequently, because of the competition for these limited hydrocarbon feedstocks, the cost of carbon black has tended to rise steeply. The search for cheaper alternatives to carbon black as a filler has thus been stimulated by the increasing costs of that material.
Synthetic calcium carbonates have attracted interest since their very small ultimate particle size suggests that they may be capable of providing a high degree of reinforcement in elastomer compositions. One advantage of these calcium carbonates is that they can be manufactured by processes that are not high consumers of energy and that involve raw materials of low cost. Thus, synthetic calcium carbonate of high surface area and of regulated particle size, shape and distribution and crystal structure can be prepared by the carbonation of an aqueous lime suspension. Another advantage is that the use of calcium carbonate as a filler permits the production of colored or white elastomer composites.
However, as further discussed in GB No. 2,131,435A, in order to achieve adequate reinforcement, there should be a strong interaction between the surface of the filler particles and the elastomer, leading to adhesion. Because of the hydrophilic nature of the surface of precipitated calcium carbonates (in contrast to carbon black) adhesion at the filler/elastomer interface is poor.
The surface treatment of precipitated calcium carbonate using stearic acid has been practiced for many years. Although such a treatment increases the hydrophobicity of the calcium carbonate surface, the improvement in adhesion at the filler/elastomer interface is not significant and the performance characteristics of stearate-coated grades of precipitated calcium carbonate as rubber-reinforcing fillers are only modest.
In Canadian patent No. 1,110,504 issued to Imperial Chemical Industries, it is disclosed that the compatibility of a basic particulate filler with an organic polymer can be improved by coating the filler with an organic polymer which contains at least one unsaturated group and a carboxylic acid or carboxylic anhydride group. Such coated fillers are stated to be especially useful as fillers in natural or synthetic rubbers. The filler may be prepared by mixing a suspension of precipitated calcium carbonate with the triethylammonium salt of the methyl half-ester of a polybutadiene-maleic anhydride adduct.
In U.S. Ser. No. 943,669 filed Dec. 17, 1986, the preparation is described of a filler, useful in polymer systems, wherein a layered lattice silicate such as kaolin in substantially dry, particulate form is contacted with an organic monomer, co-monomers or a prepolymer, in the presence of gaseous hydrogen. Surface polymerization or reaction in situ on the silicate takes place and it was found that with the use of hydrogen, strong bonding of modifier to the silicate surface is achieved.
In Iannicelli U.S. Pat. No. 3,567,680 a finely divided particulate inorganic reinforcing filler pigment for use in elastomers is disclosed consisting essentially of kaolin clay surface modified with a mercaptoorganosilane, e.g., mercaptopropyltrimethoxysilane. See also Papalos U.S. Pat. No. 3,227,675. The present invention, however, avoids the use of the expensive organosilanes.
Kamagaito et al U.S. Pat. No. 4,472,538 again uses a silane compound. A composite material is provided by contacting a clay mineral having laminated aluminum silicate layers with an organic monomer to allow the organic monomer to be adsorbed on or intercalated between the aluminum silicate layers, and contacting the clay mineral with a silane compound having 2 to 4 chlorine atoms such as dichlorodimethylsilane to promote polymerization of the organic monomer, thereby forming a composite material composed of a clay mineral having an organic high polymer absorbed thereon or intercalated therebetween. The monomer may be a vinyl monomer such as styrene, isoprene or vinyl acetate. Example 2 shows use of E-caprolactam as the monomer and use of the product in a nylon 6,6 matrix.
Takahashi et al in U.S. Pat. No. 3,773,708 describe a method for forming a filler resin or rubber composition which comprises crushing an inorganic filler wherein the filler is reduced in size by an average of more than 10 times the original diameter in the presence of a free radical polymerizable or ionic polymerizable monomer whereby the monomer is polymerized and is bonded to the newly formed surfaces of the crushed filler to form a modified filler, and admixing said modified filler into a matrix of a thermoplastic resin, thermosetting setting resin or rubber. The patentees use a crushing technique to form new surfaces that help to initiate polymerization via free radical or ionic polymerization. They recite that mechanical crushing of the inorganic filler will result in exposing of newly formed surfaces which will momentarily contain active radicals or ions due to the shear and impact forces which break chemical bonds in the particle. On the other hand, the process of the present invention gives controlled reaction on each particle at the desired particle size distribution for a particular application. Therefore, these is substantially no change in the initial particle size distribution of the mineral during the process.
As described in Canadian patent No. 1,119,142 issued to Anglo-American Clays Corp., naturally occurring calcium carbonate is found in natural calcite deposits that are highly contaminated with discolorants which contribute to the discoloration of the otherwise relatively colorless calcium carbonate. These contaminants may include pyrites, mica and siliceous materials, e.g., sand and quartz. The process of the patent for producing a high brightness particulate calcium carbonate comprises in sequence the steps of coarse-milling the natural calcitic ore; subjecting the coarse-milled product as an aqueous slurry including less than 40% solids to a froth flotation, and separating with the froth discoloring contaminants; dewatering the underflow product from the froth flotation to at least 60% solids by weight; and wet-milling the dewatered product to an output product of a specified E.S.D. (equivalent spherical diameter) and brightness. This patent indicates what is meant by the term "wet processing" or "wet grinding". In Example I a sample of curshed ore was "wet ground" in slurry form at 65% solids by use of a ball mill to provide particulate material of a certain E.S.D. A dispersing agent was used during ball milling in order to facilitate grinding - in this example Dispex N40, a trademark for a sodium polyacrylate composition, was used as a dispersant. By contrast, a dry ground or dry processed carbonate is one which has been ground in dry condition without the use of dispersants.
U.S. Pat. No. 3,604,634 is further elucidative of dry ground and of wet processed calcium carbonates. The dispersing agents sold under the trademark Dispex include the sodium, potassium and ammonium salts of polyacrylic acids and of polymethacrylic acids.
According to the invention, in the modification of a substrate filler material such as a calcium carbonate by surface treatment thereof with a polymerizable material, e.g., a monomer, co-monomers or a prepolymer, the presence of a flowing gas containing hydrogen activates the surface and makes it reactive towards the polymerizable species. However, in one aspect of the invention, when the carbonate has been subjected to wet processing,, i.e., has been ground in an aqueous slurry containing a dispersing agent, a problem that arises is that the dispersing agent covers the active sites resulting from the use of hydrogen and renders the carbonate inactive. The solution of this problem is an object of the invention.